Today's Mercury Flyby To Be the First Since 1974

Adding to the picture of a very different kind of planet, it takes Mercury 88 Earth days to circle the sun and six months to rotate around its axis to make a full "day."

When Mariner 10 passed by Mercury in 1974 and 1975, it was able to capture images of only about half the planet's surface. Messenger is expected to photograph the entire planet before it is done.

Today's flyby will include some anxious moments as the spacecraft passes behind Mercury, going without solar power for 14 minutes and out of radio contact for at least 48 minutes. About 55 hours after the probe's instruments were turned on yesterday, data will begin to stream back to Earth. The instruments will take more than 1,200 images during the flyby and will fill the onboard recorder with more than 700 megabytes of data, said systems engineer Eric Finnegan of the Johns Hopkins University Applied Physics Laboratory in Laurel.

"Fifty minutes prior to closest approach, signals from the spacecraft will go quiet as Messenger passes behind Mercury, out of Earth's view," he said. "Forty-eight minutes later, engineers and scientists on the ground will attempt to witness the gravitational pull of the planet firsthand by reacquiring the transmitted signal from the spacecraft within minutes of the closest approach point."

As of today, Messenger is only slightly more than halfway through its journey, which will ultimately cover almost 5 billion miles. It will make its next close approach to Mercury in October.

Asked why anyone might care about the characteristics and dynamics of Mercury, Solomon, the principal investigator, gave two reasons: "natural human curiosity" and because the inner planets -- the "siblings of Earth" -- have special meaning to people.

"We are the shepherds of our planet, and part of that role is understanding what makes it work," Solomon said. "Well, Mercury was formed at about the same time as the other siblings, under common processes, but it resulted in quite a different outcome. Quite directly, the depth of our understanding of Earth is taxed by our inability to explain other outcomes, and so learning about Mercury is an extension of understanding how Earth began and how it operates."